What is a Calorie?

The calorie equals exactly 4.184 joules (the 'thermochemical calorie') and is the historical unit for heat in chemistry and physics. It was originally defined as the heat needed to raise 1 gram of water by 1°C (a definition that varied slightly with starting temperature, hence multiple 'calories' — 4.184 J was set as the exact thermochemical convention). The calorie is widely used in older physics and chemistry literature for reaction energies, specific heat capacities, and thermodynamic calculations. Note: this 'small calorie' (lowercase c) is 1/1000 of the food Calorie (capital C) used in nutrition. The chemistry calorie relates to the joule (4.184 J = 1 cal), the kilocalorie (1,000 cal = 1 kcal = 1 food Calorie), and the BTU (1 BTU ≈ 252 cal). Modern SI usage in scientific publications has largely replaced the calorie with the joule, but it persists in medical and chemistry contexts.

Visual reference for how the calorie relates to other energy units. Each row links to the full converter for that pair.

• Thermodynamics and older physics texts
• Chemistry energy calculations
• Some engineering heat-transfer contexts

• ✓1 cal = 4.184 J. 1 kcal = 4184 J.
• ✓Scientific "calorie" and nutrition "Calorie" differ by 1000×.
• ✓Prefer joules in modern scientific writing.

• ✗Assuming "calories" on food labels means gram calories — they are kcal.
• ✗Mixing gram calories and kilocalories in calculations.
• ✗Confusing calorie (energy) with calorimeter (apparatus).